This article explains how to create an Ethernet DHCP (also known as Cable modem(DHCP)) interface on Sophos UTM.
This is a standard Ethernet interface that will obtain IP address settings automatically, using DHCP. This interface type is very commonly used for residential or SOHO internet connections.
Use this interface type when you are:
- Connecting to a standard internet connection, and the ISP has not issued a permanent IP address
- Connecting to a network that offers DHCP
- The network you are connecting to requires PPP type authentication
Do NOT use this interface type if:
- No DHCP server is available on the network you are connecting to.
This article focuses on a single interface type. To learn more about other interface types, please view article #118899
Known to apply to the following Sophos product(s) and version(s)
What To Do
Interfaces may be added and edited in the WebAdmin interface under: Interfaces & Routing > Interfaces > Interfaces.
- Click the New Interface button to add a new interface.
- Then in the Type field, select: Ethernet DHCP (Cable Modem(DHCP) in older firmware versions)
- The following image and table outline the options available, and how to fill them out:
|Interface Image ||Configuration Options |
| || |
Name: This is the name that will be shown throughout the UTM configuration, wherever this interface is referenced. Names that indicate, or are specific to, purpose, are often the most useful. For example names like: Internal, DMZ, Guest, External. or External-XO, External-ATT, are purpose specific names that make it clear to someone who didn't create the object, what purpose it serves.
Hardware: Unused Ethernet ports will be shown here. If none are listed, then all interfaces are presently configured. UTM appliances configure eth3 by default, for HA/Clustering. If HA is not needed, and HA is not disabled, eth3 may be made available on UTM appliances, by disabling HA under Management | High Availability | Configuration.
IPv4 Default GW: If the interface is a connection to the internet, then a default gateway IP may be set by enabling this option. If this is not an internet connection, then this option does not normally need to be enabled.
Comment:(Optional) This is a free text field, where brief comments may be recorded\
Advanced Options Hostname:(Optional) This hostname will be reported to the DHCP server. If the DHCP server requires or expects a specific hostname value, it may be set here. Otherwise, it is normally left blank.
MTU: The default value of 1500 is the standard value for internet traffic. It may be apropriate to change this when connecting to internal networks using jubmo frames, or to ISPs that require smaller MTU values.
Proxy Arp: This should normally be left disabled, though it may be used in special circumstances, to allow hosts on one network to resolve the MAC address of machines on another network.
Displayed max:(Optional) The displayed max value is a numeric field that is used for display purposes only. Entering the correct value in this field to match the available bandwidth provided on this link, allows the UTM to show accurate percentage graphs for each interface on dashboard. This has no impact on network behavior, as it is only used for display purposes.
The interface type was renamed from Cable Modem(DHCP) to Ethernet DHCP in Sophos UTM firmware version 9.100